Diseases of the major circulatory and renal organs and vessels have created a need for prosthetic grafts to bypass, repair and/or replace the function of the diseased organs and vessels. Such grafts should ideally be non-immunogenic, non-calcific, and readily capable of recreating or reestablishing the natural blood contact interface of the organ or vessel to be replaced or repaired. Complications that have inhibited the widespread use of prosthetic grafts in organs and vessels in contact with blood include: (1) intimal hyperplasia, whereby smooth muscle cell and myofibroblast proliferation and extracellular matrix accumulation cause thickening of the intima in the graft and in the adjoining vessels, and ultimately lead to failure of the graft; and, (2) occlusion of the graft, whereby platelet adhesion and activation at the lumenal surface of the graft initiates thrombosis which, particularly in smaller bore vessel grafts, typically leads to complete occlusion of the graft.
Research over the past several decades has yet to produce a synthetic or biosynthetic small bore vascular graft which can approach the patency rates of autologous vessels. Since small bore grafts have a higher surface area to volume ratio and lower flow rates than larger grafts, the interaction of the graft with the blood is much greater. Platelet adhesion and activation at the lumenal surface of the graft are much more likely to result in complete graft occlusion. Larger vascular grafts are able to remain patent despite a layer of clot lining the lumen because this layer of clot undergoes constant remodeling and essentially maintains a constant thickness. In contrast, clotting on the surface of a graft smaller than 6 mm in inner diameter has a snowball effect and results in a continuous growth of the surface clot until the entire graft is occluded.
Currently, non-synthetic or biological small bore grafts are routinely used as an arterial replacement since nothing has proven to perform nearly as well as the autologous saphenous vein or internal mammary artery, which are the conventional biological materials used as a small diameter vascular graft. The use of these vessels requires additional surgery, particularly in the case of the saphenous vein, whereby the entire length of the leg must be opened to remove the vessel. The harvesting surgery increases the total operating time and can also lead to complications and discomfort. Furthermore, a small percentage of patients do not have autologous vessels suitable for harvesting. In some cases, the vessels are not available due to previous surgery, while in other cases, the vessel may be too small or varicose.
Even larger bore vessel and organ prosthetic grafts, however, suffer from complications associated with smooth muscle proliferation, compliance mismatch with native vessels, and poor endothelialization due to blood shear stresses and mechanical damage. Therefore, researchers have focused much effort on the development of bioinert and hemocompatible graft materials. However, a completely non-fouling surface has yet to be discovered and many now view the quest for such a material as unrealistic.
Rather than creating a non-fouling surface, others have focused on recreating the natural blood contacting interface in the body by seeding vascular grafts with endothelial cells (See for example, U.S. Pat. No. 5,723,324 to Bowlin et al.; U.S. Pat. No. 5,674,722 to Mulligan et al., U.S. Pat. No. 5,785,965 to Pratt et al., U.S. Pat. No. 5,766,584 to Edelman et al.). Although a small number of grafts seeded lumenally with endothelial cells have been implanted clinically outside of the United States, and improved patencies over non-seeded grafts have been observed, this approach has generally enjoyed mixed success, and the concept still faces many challenges. First, it is necessary that the cells used to seed the graft be autologous or otherwise non-immunogenic to avoid recognition and destruction of the cells by the patient's immune system. To obtain autologous endothelial cells from a patient, the cells must be harvested from an isolated blood vessel. The harvesting surgical procedure not only increases prosthetic implant preparation time, but can also lead to complications and discomfort for the patient.
Second, retention of the cells on the graft surface after implantation has been an issue. A number of methods have been disclosed to address this issue, and include forcible injection of endothelial cells into the graft, preclotting and seeding the lumenal surface of the graft, static adhesion-seeding of the lumen, vacuum seeding of the lumen, seeding the lumen in an extracellular matrix, and seeding of the lumen using electrostatic and gravitational forces. These methods are reviewed or disclosed in more detail in U.S. Pat. No. 5,723,324, ibid. Additionally, it has been suggested that flow conditioning the seeded graft in vitro prior to implantation would improve cell retention by allowing the cells to secrete adhesion factors in response to slowly increasing shear rates (Dardik et al., 1999, J Vasc Surg 29: 157–67; Ballerman et al., 1995, Blood Purif 13: 125–34; and Ott and Ballerman, 1995, Surgery 117: 334–9). Although there is some evidence that methods such as conditioning may improve cell retention, all of these methods add yet another level of complexity to the seeding process and it is still not clear that significantly improved cellular retention can be achieved.
Therefore, there is a need for prosthetic grafts for use in the repair and replacement of vessels and organs in contact with blood flow that have improved long term patency and success rates, and which reduce the stress and discomfort experienced by the patient.